Ovarian cancer largely is confined to the peritoneal cavity for much of its natural history (Runowicz, C. D. (2008) Cancer J. 14, 7-9). Peritoneal mesothelioma is a highly invasive tumor originating from the mesothelial linings of the peritoneum (Palumbo, et al., (2008) Curr. Med. Chem. 15, 855-867). The development of effective drug regimens against ovarian cancer and mesothelioma has proven extremely difficult.
Mesothelin was first identified in 1992 by the monoclonal antibody (mAb) K1 that was generated by the immunization of mice with human ovarian carcinoma (OVCAR-3) cells (Chang, et al., (1992) Int. J. Cancer 50, 373-381). It is a glycosyl-phosphatidylinositol (GPI)-anchored glycoprotein present on the cell surface (Chang, K., and Pastan, I. (1996) Proc. Natl. Acad. Sci. USA 93, 136-140). The mesothelin gene encodes a 71-kDa precursor protein that is processed to a 40-kDa GPI-anchored protein termed mesothelin. Mesothelin is a differentiation antigen that is present on a restricted set of normal adult tissues such as the mesothelium. In contrast, it is overexpressed in a variety of cancers including mesothelioma, ovarian cancer, and pancreatic cancer (Hassan, R., and Ho, M. (2008) Eur. J. Cancer 44, 46-53). It has been demonstrated that mesothelin is also expressed on the surface of non-small cell lung cancer cells (Miettinen, M., and Sarlomo-Rikala, M. (2003) Am. J. Surg. Pathol. 27, 150-158; and Ordonez, N. G. (2003) Am. J. Surg. Pathol. 27, 1418-1428), especially most lung adenocarcinomas (Ho, et al., (2007) Clin. Cancer Res. 13, 1571-1575). We and others have shown that mesothelin is shed from tumor cells (Hellstrom, et al., (2006) Cancer Epidemiol. Biomarkers Prev. 15, 1014-1020; and Ho, et al., (2006) Cancer Epid. Biomarkers Prev. 15, 1751). Shed serum mesothelin has been approved by the Federal Drug Administration (FDA) as a new diagnostic biomarker in mesothelioma. We have found that antibodies specific for mesothelin are elevated in the sera of patients with mesothelioma and ovarian cancer, and that this elevation is associated with high expression of mesothelin in tumors (Ho, et al., (2005) Clin. Cancer Res. 11, 3814-3820). In a phase I clinical study of an intrapleural IFN-β gene transfer using an adenoviral vector in patients with mesotheliomas, antitumor immune responses targeting mesothelin were elicited in several patients (Sterman, et al., (2007) Clin. Cancer Res. 13, 4456-4466). A recent study indicated that anti-mesothelin antibodies and circulating mesothelin relate to the clinical state in ovarian cancer patients (Hellstrom, et al., (2008) Cancer Epidemiol. Biomarkers Prev. 17, 1520-1526).
Pastan and colleagues developed an immunotoxin (SS1P) with a high affinity Fv for mesothelin (Pastan, et al, (2006) Nat. Rev. Cancer 6, 559-565). Two phase I clinical trials were completed at the National Cancer Institute (NIH, Bethesda, Md.) and there was sufficient antitumor activity of SS1P to justify a phase II trial. MORAb-009 is a high-affinity chimeric (mouse/human) monoclonal IgG1/κ containing the same Fv (SS1) for mesothelin. A Phase I clinical trial of MORAb-009 has been initiated in patients with ovarian cancer, mesothelioma, pancreatic cancer, and non-small cell lung cancer (Hassan, et al., (2007) Cancer Immun. 7, 20).
There is evidence that in mouse mammary epithelial cells activation of the Wnt signaling pathway can lead to an increase in mesothelin expression (Prieve, M. G., and Moon, R. T. (2003) BMC Dev. Biol. 3, 2). This may well explain the fact that mesothelioma and ovarian cancer with constitutive activation of Wnt signaling have high mesothelin expression. The biological functions of mesothelin remain elusive. The mesothelin knockout mice did not have a detectable phenotype (Bera, T. K., and Pastan, I. (2000) Mol. Cell. Biol. 20, 2902-2906).
Mucins are heavily glycosylated proteins found in the mucus layer or at the cell surface of many epitheliums (Desseyn, J. L., Tetaert, D., and Gouyer, V. (2008) Gene 410, 215-222). There are two structurally distinct families of mucins, secreted and membrane-bound forms. CA125 (also known as MUC16) was first identified in 1981 by OC125, a mAb that had been developed from mice immunized with human ovarian cancer cells (Bast, et al., (1981) J. Clin. Invest. 68, 1331-1337). The first cDNA clones were reported in 2001 (Yin, B. W. T., and Lloyd, K. O. (2001) J. Biol. Chem. 276, 27371-27375; and O'Brien, et al., (2001) Tumour Biol. 22, 348-366). It is a very large membrane-bound cell surface mucin, with an average molecular weight between 2.5 and 5 million Dalton. It is also heavily glycosylated with both O-linked and N-linked oligosaccharides (O'Brien, et al., (2002) Tumour Biol. 23, 154-169). It is shed into the serum and is used for monitoring response to therapy in ovarian cancer (Bast, et al., (1983) N. Engl. J. Med. 309, 883-887). The peptide backbone of CA125 is composed of the N-terminal region, extensive Ser/Thr/Pro-rich tandem repeats (TR) with 156 amino acids each with both N- and O-glycosylations, a SEA domain with high levels of O-glycosylation and a C-terminal region with a short cytoplasmic tail (O'Brien, et al., (2001) Tumour Biol. 22, 348-366). The SEA domain was first identified as a module commonly found in sea urchin sperm protein, enterokinase and agrin (Bork, P., and Patthy, L. (1995) Protein Sci. 4, 1421-1425; and Maeda, et al., (2004) J. Biol. Chem. 279, 13174-13182). The significance of the SEA domain in CA125 is not clear.
CA125 was originally used as a biomarker in ovarian cancer due to its high expression in ovarian carcinomas. A majority (88%) of mesotheliomas are also CA125 positive on the cell membrane (Bateman, et al., (1997) Histopathology 30, 49-56). It was shown that 25% of peritoneal mesotheliomas have high CA125 expression (Attanoos, et al., (2002) Histopathology 40, 237-244). The intensity of CA125 membranous expression is indistinguishable between ovarian carcinomas and peritoneal mesotheliomas. Gene expression analysis using SAGE tag database has shown that mesothelioma has the second highest co-expression of CA125 and mesothelin after ovarian cancer (Rump, et al., (2004) J. Biol. Chem. 279, 9190-9198). Rump and colleagues have shown that mesothelin binds to CA125 and that this interaction may mediate cell adhesion (Rump, et al., supra). Since mesothelin is present on peritoneal mesothelium, there may be an important role for the mesothelin-CA125 interaction in tumorigenesis of ovarian cancer or mesothelioma in the peritoneal cavity. Scholler et al. recently showed that CA125/mesothelin-dependent cell attachment could be blocked with anti-CA125 antibodies (Scholler, et al., (2007) Cancer Lett. 247, 130-136). The mesothelin binding site on CA125 may lie within the 156 amino acid TR units, indicating multimeric binding of mesothelin to CA125. It has been found that the extraordinarily abundant N-glycans on CA125, presumably in the TR region, are required for binding to both glycosylated and non-glycosylated mesothelin (Gubbels, et al, (2006) Mol. Cancer. 5, 50).
There remains a need for pharmacological agents that effectively disrupt binding between CA125 and mesothelin and cell adhesion mediated by the CA125/mesothelin interaction.